Which best explains why on ization energy tends to decrease from the top to the bottom of a group

ionization energy is the amount of energy it takes to remove an atoms most loosely bound electron. going from the top of a group to the bottom, ionization energy decreases, as it is getting easier and easier to remove the outermost electron. As you move down a group, you add principle energy levels, which causes shielding. this means that there is decreased communication between the outermost electron and the nucleus, making it easier to remove the outer electron.